Putting all these forms of human achievement together--the extension of the scientific knowledge of Nature with consequent mastery over her, the scientific knowledge of social phenomena in the past and the present, the records of philosophic speculation, the mass of literary and artistic products, the establishment, however partial and imperfect, of regulative moral and political principles--it will be seen that the accumulation of this vast stock of intellectual wealth has been an even more important factor than the increase of population in giving man strength and dignity over against Nature, and in opening up to him an endless variety of modes of enjoying life--that is to say, of making it yield to him the most which its shortness and his own physical infirmities permit. The process by which this accumulation has been carried along is the central thread of history. The main aim of a history of the world must be to show what and how each race or people has contributed to the general stock. To this aim political history, ecclesiastical history, economic history, the history of philosophy, and the history of science, are each of them subordinate, though it is only through them that the process can be explained. In these last few pages intellectual progress has been considered apart from the area in which it has gone on, and apart from the conditions imposed on it by the natural features of that area. A few words are, however, needed regarding its relation to the surface of the earth. The movement of civilisation must be considered from the side of space as well as from that of time. [Sidenote: Contraction of the World] Space is a material element in the inquiry because it has divided the families of mankind from one another. Some families, such as the Chinese and the Peruvians, have developed independently, some, such as the South and West European peoples, in connection with, or perhaps in dependence on, the development of other races or peoples. Hence that which each achieved was in some cases achieved for itself only, in other cases for its neighbours as well. The contributions made by different races have--at any rate during the last four thousand years, and probably in earlier days also--been very unequal; yet none can have failed to contribute something if only by way of influencing the others. Inequality in progress would seem to have become more marked in the later than in the earlier periods. Indeed, some races, such as those of Australia, appear during many centuries, possibly owing to their isolation, to have made no progress at all. They may even have receded. When we regard the evolution and development of man from the side of his relations to space, three facts stand out--the contraction of the world, the overflow of the more advanced races, and the consequent diffusion all over the world of what is called civilisation.